Electron Confinement in Structurally Constrained σ-Bonded π-Systems. An Experimental and Density Functional Investigation

We have explored the effect of resonance overlap on electronic communication in σ-bonded π-systems through a combined experimental and computational investigation. The system studied consists of a series of diphenyl-substituted acenaphthofluoranthenes and fluoranthenes with constrained dihedral angles between polycyclic core and phenyl substituents of 90°, 70°, and 15°. The spin density distribution within the radical anions of these conformationally restricted polycyclic aromatic hydrocarbons was assessed via DFT-B3LYP calculations validated through SEEPR-derived experimental hyperfine coupling constants (hfc's). Orthogonal subsystems were essentially insulated, while substantial delocalization of spin density was observed for near-planar subsystems. The insulation of the phenyl substituents combined with the possibility of forming ladder-type polymers makes the orthogonal diphenyl-substituted acenaphthofluoranthenes intriguing building-blocks for molecular electronics.